With the introduction of high degree port count reconfigurable Wavelength Selective Switching (WSS) modules and other Reconfigurable Optical Add/Drop Multiplexing (ROADM) devices in mesh optical networks, and with the potential of high-mesh connectivity, control of optical networks is becoming very complex in terms of sequencing, messaging, and holding off control cycles for spectral stability between optically connected control domains. As new active optical line elements are introduced in a network, that are increasingly trying to control the same optical signals from upstream to downstream, stability of end-to-end optical signals is a primary concern. Conventionally, in optical networks, controllers are used to control the optical spectrum of various optical links in the optical network. Cascaded controllers are known to be unstable when operated independently without special treatment. To deal with such problems, conventional techniques use peer-to-peer messaging between neighboring sections or logical control domains that allow complete sequential operations from one control domain to the next, to ensure a stable system response.
However, there are multiple issues with peer-to-peer messaging. First, peer-to-peer messaging generate traffic overhead for control messaging. The peer-to-peer messaging framework works fine when there are a few dependent control domains. With the increase in the number of dependent control domains, especially in metro/mesh optical networks, and with the new high degree capable of ROADM architectures, the management of peer-to-peer messaging and sequencing actions between control domains, where each domain can have multi-degree upstream and downstream dependent domains, becomes very complex and unmanageable. Also, the peer-to-peer messaging does not scale in case of linear networks with long chains of cascaded control domains, where peer-to-peer messaging between two neighboring control domains fails to stop further downstream control domains that start to compensate for uncompensated upstream faults almost at the same time and generate spectrum instability.
Stability becomes an issue when peer-to-peer messaging fails to stop multiple control domains from reacting simultaneously due to capacity changes, fault handling, and performance optimization activities, collectively events in the optical network. When there are events, in a cascaded linear system, cascaded controllers can sequence operations to control optical spectrum at the same time in order to optimize launch powers and Optical Signal-to-Noise Ratio (OSNR) following the events. However, this can lead to oscillations or ringing in the optical spectrum, which in turn, can cause traffic hits. In a mesh optical network, or in a ring, where each optical section works on the same optical payloads as are in its upstream or downstream, the created oscillations over the spectrum, following events, can easily generate prolonged instability in the system.
Having independent sectional controllers or logical control domains that can control multiple actuators within a domain to maintain OSNR integrity over the optical spectrum following an event, such as an upstream fiber fault or span loss variations, remains a challenge, where if multiple downstream controllers start compensating for an upstream fault, it creates major instability over the optical spectrum. Again, conventional techniques of maintaining spectrum stability include either via peer-to-peer messaging between neighboring control domains and/or using arbitrary wait times in downstream control domains allowing upstream controller enough time to compensate for the fault or error. Other options may include designing subsequent controllers to update at the order of magnitudes slower than the upstream controllers. However, such conventional techniques are not scalable, especially in mesh optical networks, when each sectional controller may have to deal with multi-degree upstream and downstream dependent controllers, or in case of linear networks with long chain of cascaded control domains, where downstream controllers start to compensate for an uncompensated upstream fault after certain period. Hence, the challenge to achieve independent control domains with stable system response remains unresolved, which is primarily addressed herein.
The problem becomes more pronounced during a capacity change (add/delete) in network configurations in optical sections, where optical add/drop multiplexers (OADMs) are equipped with limited or no per channel actuator capabilities, and capacity change is mostly done either via upstream ROADMs equipped with Wavelength Selective Switches (WSSs) and/or by using transponder actuation capabilities. As it often happens with a capacity change, the spectral loading changes over the optical spectrum, it creates dynamic and static offsets on other in-service channels that almost linearly grows with the number of Erbium Doped Fiber Amplifiers (EDFAs) present in the line system. Although most of the EDFAs can compensate for the dynamic portion of the transient offsets using their fast gain controlled loops, it is the static offsets, that are mostly left over primarily due to spectral hole burning within the EDFAs and to some extents by Stimulated Raman Scattering (SRS), and amplifier tilt and ripple, and have to be dealt with upper layer of controllers such as sectional controller that typically operates in much slower extent (in seconds) by changing gain of the EDFAs to compensate offsets on in-service channels. And when multiple sectional controllers react simultaneously to a capacity change controlled from upstream, it generates instability in the system.
A typical example of such upstream controlled capacity change appears in flexible grid systems, specifically in a spectrally-tied super channel (also known as media channel) expansion/contraction scenario. In such case, in order to add new sub-carriers (also known as network media channels) into an already in-service super channel, the bandwidth of the super channel needs to be extended first on the flexible grid capable WSSs almost at the same attenuation level as the other in-service sub-carriers in order to avoid potential filter roll-off penalty for the new and existing sub-carriers. The allocation of the bandwidth and the expansion of the media channel should have to be done from ingress to egress in every ROADM sites before the addition of the new sub-carriers, which then have to be taken place using actuation capabilities available on the transponders. Such capacity additions or deletions over multiple OADM sections with long chain of amplifiers generate static power spectral density (PSD) offsets on other in-service channels, and without some sectional controllers that can not only just minimize PSD offsets autonomously, but also can maintain spectrum stability, the overall operation of media channel expansion/contraction in flexible-grid configuration is going to be complex and time consuming in terms of network-wide sequencing and messaging between all optically connected control domains.
Such challenges require autonomous and self-correcting controllers that can ensure the end-to-end stability of the optical spectrum, and yet handle capacity changes and faults with guaranteed optimal performance in a timely manner. With the evolving complexity of optical networks, it would be advantageous to move away from the sequential, peer-to-peer messaging-based approaches towards a non-peer-to-peer messaging-based, autonomous controller framework.